Method and apparatus for modulating a light beam

ABSTRACT

A modulator for modulating incident rays of light, the modulator having a plurality of equally spaced apart elements, each of which includes a light reflective planar surface. The elements are arranged parallel to each other with their light reflective surfaces parallel to each other. The modulator includes means for supporting elements in relation to one another and means for moving particular ones of the elements relative to others so that the moved elements transit between a first configuration wherein the modulator acts to reflect the incident rays of light as a plane mirror, and a second configuration wherein the modulator diffracts the light reflected therefrom. In operation, the light reflective surfaces of the elements remain parallel to each other in both the first and the second configurations. The perpendicular spacing between the reflective surfaces of respective elements is equal to m/4 times the wavelength of the incident rays of light, wherein m=an even whole number or zero when the elements are in the first configuration and m=an odd whole number when the elements are in the second configuration.

This application is a divisional application of U.S. patent applicationSer. No. 08/591,231, filed Jan. 18, 1996, still pending which is acontinuation-in-part of U.S. patent application Ser. No. 08/404,139,filed Mar. 13, 1995, and issued as U.S. Pat. No. 5,677,738 on Oct. 14,1997, which is a division of U.S. patent application Ser. No.08/062,688, filed May 20, 1993 and issued as U.S. Pat. No. 5,459,610 onOct. 17, 1995, which is a continuation-in-part of U.S. patentapplication Ser. No. 07/876,078, filed Apr. 28, 1992 and issued as U.S.Pat. No. 5,311,360 on May 10, 1994. This application only claims anddiscloses information found in the first filed application in this chainof pendency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus for modulating a lightbeam and more particularly to the use of a reflective, deformablediffraction grating for performing such modulation.

2. Brief Description of the Prior Art

Devices which modulate a light beam, e.g. by altering the amplitude,frequency or phase of the light, find a number of applications. Anexample of such a device is a spatial light modulator (SLM) which is anelectronically or optically controlled device which consists of one ortwo-dimensional reconfigurable patterns of pixel elements, each of whichcan individually modulate the amplitude, phase or polarization of anoptical wavefront.

These devices have been extensively developed, particularly forapplications in the areas of optical processing and computing. They canperform a variety of functions such as: analog multiplication andaddition, signal conversion (electrical-to-optical,incoherent-to-coherent, amplification, etc.), nonlinear operations andshort term storage. Utilizing these functions, SLMs have seen manydifferent applications from display technology to optical signalprocessing. For example, SLMs have been used as optical correlators(e.g., pattern recognition devices, programmable holograms), opticalmatrix processors (e.g., matrix multipliers, optical cross-bar switcheswith broadcast capabilities, optical neural networks, radar beamforming), digital optical architectures (e.g., highly parallel opticalcomputers) and displays.

The requirements for SLM technology depend strongly on the applicationin mind: for example, a display requires low bandwidth but a highdynamic range while optical computers benefit from high response timesbut don't require such high dynamic ranges. Generally, systems designersrequire SLMs with characteristics such as: high resolution, high speed(kHz frame rates), good gray scale, high contrast ratio or modulationdepth, optical flatness, VLSI compatible, easy handling capability andlow cost. To date, no one SLM design can satisfy all the aboverequirements. As a result, different types of SIMs have been developedfor different applications, often resulting in tradeoffs.

Texas Instruments, for instance, has developed a "Deformable MirrorDevice (DMD)" that utilizes an electromechanical means of deflecting anoptical beam. The mechanical motions needed for the operation of the DMDare relatively large and, as a result, the bandwidths are limited totens of kilohertz. This device, however, gives good contrast ratios andhigh-resolution and is, furthermore, compatible with CMOS, and other lowpower technologies.

Nematic and ferroelectric liquid crystals have also been used as theactive layer in several SLMS. Since the electrooptic effect in liquidcrystals is based on the mechanical reorientation of molecular dipoles,it is to be expected that liquid crystals are faster than the DMD-typedevices. Modulators using ferroelectric liquid crystals have exhibitedmoderate switching speeds (150 μsec to 100 nsec), low-power consumption,VLSI compatible switching voltages (5-10 V), high extinction ratios,high resolution and large apertures. However, these devices suffer fromthe drawbacks of limited liquid crystal lifetimes and operatingtemperature ranges. In addition, the manufacturing process iscomplicated by alignment problems and film thickness uniformity issues.

Magnetooptic modulation schemes have been used to achieve fasterswitching speeds and to provide an optical pattern memory cell. Althoughthese devices, in addition to achieving fast switching speeds, canachieve large contrast ratios, they suffer from a low (<10%) throughputefficiency and are, therefore, often unsuitable for many applications.

The need is therefore for a light modulation device which overcomesthese drawbacks.

Beside SLMs, another area of use of light modulators is in fiber optics.Fiber optic modulators are electronically controlled devices thatmodulate light intensity and are designed to be compatible with opticalfibers. For high speed communication applications, lithium niobate(LiNbO₃) traveling wave modulators represent the state-of-the-art, butthere is a need for low power, high efficiency, low loss, inexpensivefiber optic modulators, that can be integrated with silicon sensors andelectronics, for data acquisition and medical applications.

A typical use of a modulator combined with fiber optic technology, forexample, is a data acquisition system on an airplane which consists of acentral data processing unit that gathers data from remote sensors.Because of their lightweight and electromagnetic immunitycharacteristics, fiber optics provide an ideal communication mediumbetween the processor and the sensors which produce an electrical outputthat must be converted to an optical signal for transmission. The mostefficient way to do this is to have a continuous wave laser at theprocessor and a modulator operating in reflection at the sensor. In thisconfiguration, it is also possible to deliver power to the sensor overthe fiber.

In this type of application the modulator should operate with highcontrast and low insertion loss to maximize the signal to noise ratioand have low power consumption. It should further be compatible withsilicon technology because the sensors and signal conditioningelectronics used in these systems are largely implemented in silicon.

Another use of a modulator combined with fiber optic technology is inthe monitoring of sensors that are surgically implanted in the humanbody. Here optical fibers are preferred to electrical cables because oftheir galvanic isolation, and any modulator used in these applicationsshould exhibit high contrast combined with low insertion loss because ofsignal to noise considerations. Furthermore, as size is important inimplanted devices, the modulator must be integratable with siliconsensors and electronics.

There exist no prior art devices that have the characteristicsenumerated above. Modulators based on the electro-optic, Franz-Keldysh,Quantum-Confined-Stark or Wannier-Stark effect in III-V semiconductorshave high contrast and low insertion loss, but are expensive and notcompatible with silicon devices. Waveguide modulators employing glass orepilayers on silicon, require too much area and too complex fabricationto be easily integratable with other silicon devices. Silicon modulatorsthat do not employ waveguides and that are based on the plasma effect,require high electrical drive power and do not achieve high contrast.

The need is therefore for a light modulator which can be used with fiberoptic technology with low power, high efficiency, low loss, low cost andcompatibility with multimode optical fibers and silicon technology.

SUMMARY OF THE INVENTION Objects of the Invention

Accordingly, it is an object of this invention to provide a lightmodulator which alone or together with other modulators exhibits most ofthe following characteristics: high resolution, high speed (Khz framerates), gray levels (100 levels), high contrast ratio or modulationdepth, optical flatness, VLSI compatible, easy handling capability andlow cost. A further object of this invention is to provide a lightmodulator which has a tolerance for high optical power and good opticalthroughput.

Yet another object of this invention is to provide a light modulatorwhich is compatible with CMOS technology.

Still another object of this invention is to provide a light modulatorcapable of use with fiber optic technology.

A final object of this invention is to provide a light modulator whichis capable of modulating white light to produce colored light.

Summary

Briefly a presently preferred embodiment of this invention includes amodulator for modulating incident beams of light, the modulatorcomprising a plurality of equally spaced apart grating elements, each ofwhich includes a light reflective planar surface. The elements arearranged parallel to each other with their light reflective surfacesparallel to each other. The modulator includes means for supporting theelements in relation to one another and means for moving the elementsrelative to one another so that the elements move between a firstconfiguration wherein the modulator acts to reflect the incident beam oflight as a plane mirror, and a second configuration wherein themodulator diffracts the incident beam of light as it is reflectedtherefrom. In operation, the light reflective surfaces of the elementsremain parallel to each other in both the first and the secondconfigurations and the perpendicular spacing between the reflectivesurfaces of adjacent elements is equal to m/4 times the wavelength ofthe incident rays of light, wherein m=an even whole number or zero whenthe elements are in the first configuration and m=an odd number when thebeam elements are in the second configuration.

One embodiment of this invention includes a reflective deformablegrating light modulator, with a grating amplitude that can be controlledelectronically, consisting of a reflective substrate with a deformablegrating suspended above it. In its undeformed state, with no voltageapplied between the elements of the grating and the substrate, thegrating amplitude is one half of the wavelength of the incoming light.Since the round-trip path difference between the light reflected fromthe top and bottom of the grating is one wavelength, no diffractionoccurs. When a voltage is applied between the grating elements and thesubstrate, the electrostatic force pulls the elements down to cause thegrating amplitude to become one quarter of the wavelength so thatreflections from the elements and the substrate add destructively,causing the light to be diffracted. If the detection system for thereflected light has a numerical aperture which accepts only the zeroorder beam, a mechanical motion of only one quarter of a wavelength issufficient to modulate the reflected light with high contrast.

Typically the grating is formed by lithographically etching a film madeof silicon nitride, aluminum, silicon dioxide or any other materialwhich can be lithographically etched.

The deformable grating modulator of this invention has the advantagethat it is implemented in silicon technology, using micromachining andsacrificial etching of thin films to fabricate the gratings. Circuitryfor addressing and multiplexing can be manufactured on the same siliconsubstrate and thus be directly integrated with the modulator. Directintegration with electronics is an important advantage over non-siliconbased technologies like liquid crystal and electrooptic SLMs. Moreover,the device demonstrates simplicity of fabrication and can bemanufactured with only a few lithographic steps.

A further advantage of the deformable grating modulator is that becausethe deformable grating modulator utilizes diffraction rather thandeflection of a light beam, the required mechanical motions are reducedfrom several microns (as in deformable mirror devices) to tenths of amicron, thus allowing for a potential three orders of magnitude inincrease in speed. This speed is comparable to the fastest liquidcrystal modulators, but without the device suffering the same complexityin the manufacturing process.

Still a further advantage of these devices is that the required motionof the grating elements is only one quarter of a wavelength, which meansthat elements with high resonance frequencies can be used.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed description of the preferred embodiment which isillustrated in the several figures of the drawing.

IN THE DRAWING

This invention will now be further illustrated with reference to theaccompanying drawing in which:

FIG. 1(a)-(d) are cross-sections through a silicon substrateillustrating the manufacturing process of a reflective, deformablediffraction grating according to one embodiment of the invention;

FIG. 2 is an isometric, partially cut-away view of the diffractiongrating, the manufacture of which is illustrated in FIG. 1.

FIG. 3 illustrates the operation of the grating of FIG. 2 in its"non-defracting" mode;

FIG. 4 and illustrates the operation of the grating of FIG. 3 in its"diffracting" mode;

FIG. 5 is a cross-section similar to that in FIG. 3, illustrating analternative embodiment of the grating in its "non-defracting" mode;

FIG. 6 is a cross-section similar to that in FIG. 4, illustrating thegrating in FIG. 5 in its "defracting" mode;

FIG. 7 is a pictoral view illustrating a further embodiment of thegrating;

FIG. 8 is a cross-section along line 8--8 in FIG. 7;

FIG. 9 is a graphical representation of the modulation of a laser beamby the grating of the invention;

FIG. 10 is an illustration of how the diffraction grating of theinvention can be combined with other gratings to form a complexmodulator; and

FIG. 11 illustrates the operation of the grating in the modulation ofcolored light.

DESCRIPTION OF PREFERRED EMBODIMENTS

The fabrication steps required to produce a reflective deformablegrating 10 according to this invention are illustrated in FIG. 1(a)-(d).

The first step, as illustrated in FIG. 1(a), is the deposition of aninsulating layer 11 made of stoichemetric silicon nitride topped with abuffer layer of silicon dioxide followed by the deposition of asacrificial silicon dioxide film 12 and a low-stress silicon nitridefilm 14, both 213 nm thick, on a silicon substrate 16. The low-stresssilicon nitride film 14 is achieved by incorporating extra silicon(beyond the stoichiometric balance) into the film, during the depositionprocess. This reduces the tensile stress in the silicon nitride film toroughly 200 MPa.

In the second step, which is illustrated in FIG. 1(b), the siliconnitride film 14 is lithographically patterned into a grid of gratingelements in the form of elongate elements 18. In an individual grating,all the elements are of the same dimension and are arranged parallel toone another with the spacing between adjacent elements equal to the beamwidth. Depending on the design of the grating, however, the elementscould typically be 1, 1.5 or 2 μm wide with a length that ranges from 10μm to 120 μm. After this lithographic patterning process a peripheralsilicon nitride frame 20 remains around the entire perimeter of theupper surface of the silicon substrate 16. This frame 20 is furtherillustrated in FIG. 2 and will be more fully described below withreference to that figure.

After the patterning process of the second step, the sacrificial silicondioxide film 12 is etched in hydrofluoric acid, resulting in theconfiguration illustrated in FIG. 1(c). It can be seen that each elementnow forms a free standing silicon nitride bridge, 213 nm thick, which issuspended a distance of 213 nm (this being the thickness of the etchedaway sacrificial film 12) clear of the silicon substrate. As can furtherbe seen from this figure the silicon dioxide film 12 is not entirelyetched away below the frame 20 and so the frame 20 is supported, adistance of 213 nm, above the silicon substrate 16 by this remainingportion of the silicon dioxide film 12. The elements 18 are stretchedwithin the frame and kept straight by the tensile stress imparted to thesilicon nitride film 14 during the deposition of that film.

The last fabrication step, illustrated in FIG. 1(d), is sputtering,through a stencil mask, of a 50 nm thick aluminum film 22 to enhance thereflectance of both the elements 18 and the substrate 16 and to providea first electrode for applying a voltage between the elements and thesubstrate. A second electrode is formed by sputtering an aluminum film24, of similar thickness, onto the base of the silicon substrate 16.

The final configuration of the grating is illustrated in FIG. 2. Here itcan be seen that the elements 18 together with the frame 20 define agrating which, as will be later explained, can be used for modulating alight beam. Furthermore, and as can be gathered from the above describedmanufacturing process, the frame 20 is formed integrally with theelements 18 and thus provides a relatively rigid supporting structurewhich maintains the tensile stress within the elements 18. In so doing,and as the frame 20 is supported by the remainder of the, silicondioxide film 12 that was not etched away, the elements are kept straightand a distance of 213 nm above the surface of the silicon substrate 16.

The operation of the deformable grating 10, formed by the above process,is illustrated with reference to FIG. 3 and 4. Before commencing thedescription of how the grating operates, however, it should be recalledthat, in this case, each of the elements 18 are 213 nm thick and aresuspended a distance of 213 nm clear of the substrate 16. This meansthat the distance from the top of each element to the top of thesubstrate is 426 nm. Similarly, the distance between the top of thereflective surface on the elements to the top of the reflective surfaceon the substrate is also 426 nm. This distance is known as the gratingamplitude.

In FIG. 3 the grating 10 is shown with no voltage applied between thesubstrate 16 and the individual elements 18, and with a lightwave,generally indicated as 26, of a wavelength λ=852 nm incident upon it.The grating amplitude of 426 nm is therefore equal to half of thewavelength of the incident light and, therefore, the total path lengthdifference for the light reflected from the elements and from thesubstrate equals the wavelength of the incident light. As a result,light reflected from the elements and from the substrate add in phaseand the grating 10 acts to reflect the light as a flat mirror.

However, as illustrated in FIG. 4, when a voltage is applied between theelements 18 and the substrate 16 the electrostatic forces pull theelements 18 down onto the substrate 16, with the result that thedistance between the top of the elements and the top of the substrate isnow 213 nm. As this is one quarter of the wavelength of the incidentlights, the total path length difference for the light reflected fromthe elements and from the substrate is now one half of the wavelength(426 nm) of the incident light and the reflections interferedestructively, causing the light to be diffracted, indicated as 28.

Thus, if this grating is used in combination with a system, fordetecting the reflected light, which has a numerical aperture sized todetect one order of diffracted light from the grating e.g., the zeroorder, this grating can be used to modulate the reflected light withhigh contrast.

In FIGS. 5 and 6 an alternative embodiment of the diffraction grating 30of the invention is illustrated. In this embodiment the grating 30consists of a plurality of equally spaced, equally sized, fixed elements32 and a plurality of equally spaced, equally sized, movable elements 34in which the movable elements 34 lie in the spaces between the fixedelements 32. Each fixed element 32 is supported on and held in positionby a body of supporting material 36 which runs the entire length of thefixed element 32. The bodies of material 36 are formed during alithographic etching process in which the material between the bodies 36is removed.

As can be seen from FIG. 5 the fixed elements 32 are arranged to becoplanar with the movable elements 34 and present a flat upper surfacewhich is coated with a reflective layer 38. As such the grating 30 actsas a flat mirror when it reflects incident light, however, when avoltage is applied between the elements and an electrode 40 at the baseof the grating 30 the movable elements 34 move downwards as isillustrated in FIG. 6. By applying different voltages the resultantforces on the elements 34 and, therefore, the amount of deflection ofthe movable elements 34 can be varied. Accordingly, when the gratingamplitude (defined as the perpendicular distance d between thereflective layers 38 on adjacent elements is m/4 times the wavelength ofthe light incident on the grating 30, the grating 30 will act as a planemirror when m=0, 2, 4 . . . (i.e. an even number or zero) and as areflecting diffraction grating when m=1, 3, 5 . . . (i.e. an oddnumber). In this manner the grating 30 can operate to modulate incidentlight in the same manner as the grating 10 illustrated in FIGS. 1 to 4.

Yet another embodiment of the diffraction grating of the invention isillustrated in FIGS. 7 and 8. As with the grating 10 in FIGS. 1 to 4this grating 41 consists of a sacrificial silicon dioxide film 42, asilicon nitride film 44 and a substrate 46. In this embodiment, however,the substrate 46 has no reflective layer formed thereon and only thesilicon nitride film 44 has a reflective coating 45 formed thereon. Asis illustrated in FIG. 7 the deformable elements 48 are coplanar intheir undeformed state and lie close to one another so that togetherthey provide a substantially flat reflective surface. The elements 48are, however, formed with a neck 50 at either end, which is off-centerof the longitudinal center line of each of the elements.

When a uniformly distributed force, as a result of an applied voltagefor example, is applied to the elements 48 the resultant force F, foreach element 48, will act at the geometric center 52 of that element. Aseach resultant force F is off-set from the axis of rotation 54 (whichcoincides with the centerline of each neck 50), a moment of rotation ortorque is applied to each element 48 which results in a rotation of eachelement 48 about its axis 54 to the position 48' indicated in brokenlines. This is known as "blazing" a diffraction grating.

As can be seen from FIG. 8, the reflective planes 56 of the elements 48remain parallel to each other even in this "blazed" configuration andtherefore, the grating amplitude d is the perpendicular distance betweenthe reflective surfaces of adjacent elements. This "blazed" grating willoperate to diffract light in the same manner as a sawtooth grating.

Although not illustrated in any of FIGS. 1 to 8, it will be apparentthat a deformable diffraction grating can be constructed in which, inits undeformed state, all the reflective elements are in the form ofmovable elements arranged parallel, adjacent and coplanar with eachother. In this type of grating not only the grating amplitude (i.e., theperpendicular distance between adjacent reflective surfaces) can bevaried but also the average height of all the reflective surfaces can bechanged by moving all the elements relative to a fixed datum. Thisarrangement has the advantage that both the amplitude and the phase ofthe reflected/diffracted light can be modulated.

The electrical, optical and mechanical characteristics of a number ofmodulators, similar in design to the modulator illustrated withreference to FIGS. 1 to 4 but of different dimensions were investigatedby using a Helium Neon laser (of 633 nm wavelength) focused to a spotsize of 36 μm on the center portion of each modulator. This spot size issmall enough so that the curvature of the elements in the region wherethe modulator was illuminated can be neglected, but is large enough toallow the optical wave to be regarded as a plane wave and coveringenough grating periods to give good separation between the zero andfirst order diffraction modes resulting from the operation of thegrating. It was discovered that grating periods of (i.e.) the distancebetween the centerlines of two adjacent elements in the grating, 2, 3and 4 μm and a wavelength of 633 nm resulted in first order diffractionangles of 18°, 14° and 9° respectively.

One of these first order diffracted light beams was produced by using a120 μm-long grating modulator with 1.5 μm-wide elements at atmosphericpressure together with a HeNe light beam modulated at a bit rate of 500kHz. detected by a low-noise photoreceiver and viewed on anoscilloscope. The resulting display screen 30 of the oscilloscope isillustrated in FIG. 9.

However, before proceeding with a discussion of the features illustratedin this figure, the resonant frequency of the grating elements shouldfirst be considered.

The resonant frequency of the mechanical structure of the grating of theinvention was measured by driving the deformable grating modulator witha step function and observing the ringing frequency. The area of thealuminum on the deformable grating modulator is roughly 0.2 cm², whichcorresponds to an RC limited 3-dB bandwidth of 1 MHz with roughly 100ohms of series resistance. This large RC time constant slowed down thestep function, however, enough power existed at the resonant frequencyto excite vibrations, even in the shorter elements. Although the ringingcould be observed in normal atmosphere, the Q-factor was too low(approximately 1.5) for accurate measurements, so the measurements weremade at a pressure of 150 mbar. At this pressure, the Q-factor rose to8.6, demonstrating that air resistance is the major damping mechanism,for a grating of this nature, in a normal atmosphere.

Nonetheless, it was found that due to the high tensile stress in theelements, tension is the dominant restoring force, and the elementscould therefore be modeled as vibrating strings. When this was done andthe measured and theoretically predicted resonance frequencies compared,it was found that the theory is in good agreement with the experimentalvalues, particularly when considering the uncertainty in tensile stressand density of the elements. As it is known that the bandwidth of forcedvibrations of a mechanical structure is simply related to the resonancefrequency and Q-factor, a Q-factor of 1.5 yields a 1.5 dB bandwidth ofthe deformable grating modulator 1.4 times larger than the resonancefrequency. The range of bandwidths for these gratings is therefore from1.8 MHz for the deformable grating modulator with 120 μm elements to 6.1MHz for the deformable grating modulator with 40 μm elements.

Returning now to FIG. 9, it should be noted that with an applied voltageswing of 3 V, a contrast of 16 dB for the 120 μm-long bridges could beobserved. Here the term "modulation depth" is taken to mean the ratio ofthe change in optical intensity to peak intensity.

The input (lower trace 62) on the screen 60 represents a pseudo-randombit stream switching between 0 and -2.7 V across a set of gratingdevices on a 1 cm by 1 cm die. The observed switching transient with aninitial fast part followed by a RC dominated part, is caused by theseries resistance of the deformable grating modulator, which iscomparable to a 50 ohm source resistance.

The output (upper trace 64) on the screen corresponds to the opticaloutput of a low-noise photoreceiver detecting the first diffractionorder of the grating used. The output (upper trace 64) from thedeformable grating is high when the elements are relaxed and low whenthe elements are deflected. Ringing is observed only after the risingtransient, because of the quadratic dependence of the electro-staticforce on the voltage (during switching from a voltage of -2.7 V to 0 V,the initial, faster part of the charging of the capacitor corresponds toa larger change in electrostatic force, than when switching the oppositeway). This ringing in the received signal indicates a decay close tocritical damping.

Furthermore, it was found that because the capacitance increases as theelements are pulled toward the substrate, the voltage needed for acertain deflection is not a monotonically increasing function of thisdeflection. At a certain applied voltage condition, an incrementalincrease in the applied voltage causes the elements to be pulledspontaneously to the substrate (to latch) and this voltage is known asthe "switching voltage" of the modulator. The switching voltage wasfound to be 3.2 V for gratings with 120 μm long elements and, if it isassumed that tension dominates the restoring forces, the switchingvoltage is inversely proportional to the element length and therefore,the predicted switching voltage for 40 μm long elements will be 9.6 V.

The importance of the switching voltage is that below this voltage, thedeformable grating modulator can be operated in an analog fashion,however, if a voltage greater than the switching voltage is applied tothe modulator it acts in a digital manner. Nonetheless, it is importantto note that operating the modulator to the point of contact isdesirable from an applications point of view, because as discussed abovewhen the elements are deflected electrostatically, an instability existsonce the element deflection goes beyond the halfway point. This resultsin hysteretic behavior which will "latch" the element in the downposition. This latching feature gives the modulator the advantages of anactive matrix design without the need for active components. A furtheradvantage of this latching feature is that once the element has"latched" it requires only a very small "holding voltage", much smallerthan the original applied voltage, to keep the element its latchedconfiguration. This feature is particularly valuable in low powerapplications where efficient use of available power is very important.

Finally, it was discovered that when the elements of the modulators arebrought into contact with the substrate they could stick. This can besolved by adding small ridges below the beams to reduce the contact areabetween the beams and the substrate and thereby reduce the stickingproblem.

The use of the modulator of this invention in displays requires highyield integration of individual modulator elements into 2-D arrays ofmodulator devices. The modulator devices may be comprised of multiplemodulator components such as that illustrated in FIG. 10 which shows aplurality of grating modulator components combined to form a singlemodulator unit 65 which can be used to provide a gray-scale operation.Each of the individual modulator components 66, 68, 70, 72 consist of anumber of elements and gray-scale can be obtained by addressing eachmodulator in a binary-weighted manner. The hysteresis characteristic forlatching (as described above) can be used to provide gray-scalevariation without analog control of the voltage supplied to individualgrating modulator elements.

In FIG. 11 the use of the grating, in combination with other gratings,for modulating white light to produce colored light is illustrated. Thisapproach takes advantage of the ability of a grating to separate a lightspectrum into its constituent colors. By constructing modulator devices73 including three separate red, green and blue modulation components74, 76, and 78 each with a grating designed to diffract the appropriatecolor into an optical system (not shown) a color display which is whitelight illuminated by a light beam 80 can be achieved. Although shownseparated for purposes of illustration, it will be appreciated that thethree modulation components 74, 76, and 78 could be positionedcontiguous to each other as are the components 66-72 in FIG. 10 to forma single modulator device 73. This approach is attractive for large areaprojection displays.

In summary, the reflective, deformable grating light modulator of thisinvention is a device which exhibits high resolution (40 by 40 μm² to100 μm²); high response times/large bandwidth (2 to 6 MHz); highcontrast ratio (close to 100% modulation with a 3V switching voltage);is polarization independent and easy to use. This device also hastolerance for high optical power, has good optical throughput, is simpleto manufacture, CMOS compatible, and has application in a wide range offields including use as an SLM and with fiber optic technology.

Although the present invention has been described above in terms ofspecific embodiments, it is anticipated that alterations andmodifications thereof will no doubt become apparent to those skilled inthe art. It is therefore intended that the following claims beinterpreted as covering all such alterations and modifications as fallwithin the true spirit and scope of the invention.

What is claimed is:
 1. A light modulator configured for receiving a beamof multi-frequency light and for diffracting light having a composite ofa subset of frequencies of the multi-frequency light in a predetermineddirection, the modulator comprising at least two spatially separate setsof gratings each having a different grating period such that a firstgrating diffracts light of a first frequency of light from themulti-frequency light in the predetermined direction and the secondgrating diffracts light of a second frequency in the predetermineddirection wherein the light diffracted by each set of gratings in thepredetermined direction allows perception of a color which isintermediate to colors corresponding with the first and secondfrequencies.
 2. The light modulator according to claim 1 wherein eachset of gratings comprises:a. a plurality of elongated elements, eachincluding a light reflective planar surface, the elements being arrangedparallel to each other and with the light reflective surfaces of theelements selectably lying in a single parallel plane and a plurality ofparallel planes; b. means for supporting the elements in a relation toone another such that the elements are spaced at a predetermined gratingperiod; and c. means for moving a first set of the elements in adirection normal to said planes and relative to a second set of theelements, and between a first configuration wherein said first andsecond sets of elements act to reflect the incident beam of light as aplane mirror, and a second configuration wherein said first and secondsets of elements diffract the incident beam of light as it is reflectedfrom the surfaces of the elements.
 3. The light modulator according toclaim 1, wherein each sets of the gratings reflects the beam ofmulti-frequency light as a plane mirror in a first configuration, anddiffracts the beam of multi-frequency light in a second configuration.4. A light modulator configured for receiving a beam of multi-frequencylight and for diffracting light having a composite of a subset offrequencies of the multi-frequency light in a predetermined direction,the modulator comprising at least two spatially separate sets ofgratings each having a different grating period such that a firstgrating diffracts light of a first frequency of light from themulti-frequency light in the predetermined direction and the secondgrating diffracts light of a second frequency in the predetermineddirection wherein the light diffracted by each set of gratings isdirected to the predetermined direction thereby allowing perception of acolor which is intermediate to colors corresponding with the first andsecond frequencies.
 5. The light modulator according to claim 4 whereineach grating set has a predetermined grating period, such that eachgrating period is selected to diffract light of a particular frequencyin the predetermined direction.
 6. The light modulator according toclaim 5 further comprising an optical collection system configured tocollect light diffracted from the grating sets.
 7. A light modulatorconfigured for receiving a beam of multi-frequency light and fordiffracting light having a composite of a subset of frequencies of themulti-frequency light in a predetermined direction, the modulatorcomprising at least two spatially separate sets of gratings each havinga different grating period such that a first grating diffracts light ofa first frequency of light from the multi-frequency light in thepredetermined direction and the second grating diffracts light of asecond frequency in the predetermined direction and a diffractioncontrol apparatus configured to control each of the sets of gratings toselectively diffract light wherein the light diffracted by each set ofgratings in the predetermined direction allows perception of a colorwhich is intermediate to colors corresponding with the first and secondfrequencies.
 8. The light modulator according to claim 7 wherein each ofthe sets of gratings diffracts light in a first mode and reflects lightin a second mode.
 9. A color display system comprising:a. amulti-frequency light source for forming a multi-frequency light beam;b. a plurality of optical devices, each mounted to receive the lightbeam wherein each of the optical devices is selectably configurable todiffract a particular frequency of the light beam in a samepredetermined direction for forming a plurality of diffracted beams; andc. an optical collection system mounted to receive the diffracted beamsfor forming an image.
 10. The color display system according to claim 9wherein the light source generates white light.
 11. The color displaysystem according to claim 9 wherein the optical devices are configuredto diffract a particular frequency of the light beam by each having apredetermined diffraction period selected to diffract the particular oflight in the predetermined direction.
 12. At least two modulators formodulating an incident beam of light, each comprising:a. a plurality ofelongated elements, each including a light reflective planar surface,the elements being arranged parallel to each other and with the lightreflective surfaces of the elements lying in at least one parallelplane; b. means for supporting the elements in a relation to one anothersuch that the elements in each modulator are spaced at a predeterminedgrating period; and c. means for moving a first set of the elements in adirection normal to said planes and relative to a second set of theelements, and between a first modulator configuration wherein said firstand second sets act to reflect the incident beam of light as a planemirror, and a second modulator configuration wherein said first andsecond sets diffract the incident beam of light as it is reflected fromthe surfaces of the elements wherein each modulator has a differentgrating period such that a first modulator diffracts a first frequencyof light from the incident beam of light in a predetermined directionand a second modulator diffracts a second frequency of light from theincident light in the predetermined direction for allowing perception ofa color which is intermediate to colors corresponding with the first andsecond frequencies.
 13. A color display system comprising:a. amulti-frequency light source for forming a multi-frequency light beam;and b. at least two modulators for modulating an incident beam of light,each comprising:(1) a plurality of elongated elements, each including alight reflective planar surface, the elements being arranged parallel toeach other and with the light reflective surfaces of the elements lyingin at least one parallel plane; (2) means for supporting the elements ina relation to one another such that the elements in each modulator arespaced at a predetermined grating period; and (3) means for moving afirst set of the elements in a direction normal to said planes andrelative to a second set of the elements, and between a first modulatorconfiguration wherein said first and second sets act to reflect theincident beam of light as a plane mirror, and a second modulatorconfiguration wherein said first and second sets diffract the incidentbeam of light as it is reflected from the surfaces of theelementswherein each modulator has a different grating period such thata first modulator diffracts a first frequency of light from the incidentbeam of light in a predetermined direction and a second modulatordiffracts a second frequency of light from the incident beam of light inthe predetermined direction for allowing perception of a color which isintermediate to colors corresponding with the first and the secondfrequencies.
 14. A device for modulating an incident beam of light forforming color, the incident beam including a plurality of frequencies,the device comprising:a. a plurality of diffraction gratings; and b.means for selectively configuring each grating to diffract the incidentbeam of light;wherein each diffraction grating is configured to diffracta predetermined frequency of light in a same predetermined direction forspatially directing each predetermined frequency of light diffracted byeach diffraction grating for allowing perception of color which isintermediate to colors of a frequency of light from each of thediffraction gratings.
 15. The device according to claim 14 wherein eachgrating comprises a plurality of reflective elements arranged parallelto each other for reflecting the incident beam in a first configurationand diffracting the incident beam in a second configuration.
 16. Thedevice according to claim 15 wherein the means for selectivelyconfiguring comprises means for moving a first group of reflectiveelements relative to a second group of reflective elements such that thefirst group and the second group lie in a plurality of parallel planes.17. The device according to claim 14 wherein each diffraction gratinghas a different grating period for diffracting a predetermined frequencyof light in a predetermined direction.
 18. A device for modulating anincident beam of light, the incident beam including a plurality offrequencies, the device comprising:a. a first diffraction grating fordiffracting light corresponding to a first frequency in a predetermineddirection; and b. a second diffraction grating for diffracting lightcorresponding to a second frequency in the predetermined direction,wherein the first frequency corresponds to a first color and the secondfrequency corresponds to a second color, further wherein the first andsecond frequencies are spatially directed to allow perception of a thirdcolor.
 19. The device according to claim 18 further comprising a thirddiffraction grating for diffracting light corresponding to a thirdfrequency in the predetermined direction.
 20. The device according toclaim 19 wherein the first frequency corresponds to red light, thesecond frequency corresponds to green light and the third frequencycorresponds to blue light.
 21. The device according to claim 18 whereinthe first and second diffraction gratings are independently configurableto diffract light.
 22. The device according to claim 18 furthercomprising a plurality of additional display elements for forming adisplay image.